1. Field of the Invention
The invention relates to an actuating drive for a control valve.
2. Discussion of Background
An actuating drive for actuating a control valve by means of which, for example, the steam supply to a turbine of a power station installation is controlled has, connected to the actuation rod of the control valve, a main cylinder of a piston/cylinder arrangement, the main cylinder being acted on by spring force on one side and by oil under pressure on the other. When the pressure of the oil acting on the main piston in the opening direction decreases, the spring force reliably closes the control valve so that the steam supply is interrupted. This ensures that the turbine does not run out of control if the oil pressure should ever drop. The oil pressure in a drive volume is generated by an electrohydraulic converter; this oil pressure acts on the main piston and, by means of the latter, actuates the control valve. During a motion of the control valve in the opening direction, oil is fed under pressure into the drive volume but, because this motion takes place relatively slowly, relatively small cross-sections are sufficient for supplying the oil. A closing motion of the control valve, however, has to take place at a velocity which is approximately ten times higher. This involves emptying the drive volume relatively rapidly which, however, cannot be achieved through the small oil supply cross-sections.
In addition, it has been found that because of the increase in turbine powers, the control valves--and therefore also their actuating drives--must be made larger and more powerful. A corresponding proportional increase in the actuating drives leads to arrangements which require, for their actuation, relatively large quantities of oil under pressure. It is only with difficulty that commercial valves can still deal with such quantities of oil and, in addition, the dynamics of the actuating drive also suffer with increasing size.
An actuating drive with relatively better dynamics is known from the European Patent Application 0 430 089 A1. This actuating drive has a piston/cylinder arrangement in which a drive volume acted upon in a controlled manner by oil under pressure is arranged on one side of the main piston and an oil-filled buffer volume is arranged on the other side of the main piston. In this actuating drive, an oil flow from the drive volume is released by a plate valve, which is directly attached to the piston/cylinder arrangement, through a connecting conduit of large cross-section, which is directly attached to the piston/cylinder arrangement, into the buffer volume so that the control valve can be actuated very rapidly.
If the dynamics of an existing actuating drive have to be improved in association with retrofit work, a piston/cylinder arrangement has to be built completely anew for use with the existing actuating drive and this involves a relatively substantial complication. Furthermore, there is frequently no space available for the solution proposed in the European Patent Application 0 430 089 A1 so that other and more complicated solutions have to be employed.